Carrier current transmission system



Sept. 12, 3939-- R. s. cARuTHRs CARRIER CURRENT TRANSMISSION SYSTEM Filed sept. 28, 193s El f El El T TORNE P OQ.. m w m r U ovm W u owl 3.

Patented Sept. 12, 1939 UNITED ES New York Application september-2s. lssgseril N." 232,095`

'slept conditions, as well as chang in temperiire.

it becomes necessary to make rather'elaborate provisions, fr'the maintenance' of the desired transmission levels atl all frequencies. U Y( l It isl an object of "this invention to provide ""means whereby `the whole carrier transmission may be automatically maintained at a predetermined level. 1 l W A further object'is to provide means for auto= y maticallycompensating for frequency discrirnina-V i tive variations in attenuation under abnormal weather conditions. A Y l Y Y A11-additional object is to provide a means; by whichr under abnormal frequency discriminative `attenuation onA a Wide band carrier transmission SOi'amplier'overload is prevented.

Another object isfto provide an amplifier in which the high `frequency gain may be regulated withoutV change in the low frequency gain.

When Va considerable number of carrier trans- "missions are to be transmitted over a transmis,-

mission line, as for example, an open-wire telev phone or telegraph line, so that the `frequencies transmitted cover an extensive range, it isa usual practice to-provide -automaticregulations so that' 40 the-received transmission levels will remain substantially constant in spite of variations in circuitattenuation. Such variations will result from temperature changes and more particularly in the Vcaseof'open-wire lines will be `brought about lI5 vlby exposure to such other weather conditionsas fog, rain, snow and sleet. These variations in attenuation may be `classified as (l) 'those'in which-the average attenuation over the Whole transmitted band of frequencies is changed and 50 (2) those in which the attenuationchangeis discriminative as to transmitted frequency. In gen' eral, it may be said-that' a given change in weather conditions will produce a greaterichange in line`V` Aattenuation for the upper frequencies thantforV4 At' carrier frequencies -the Yline losses goodnethddffor compensating for these two classespf attenuation change 4has been found t be, the provision of two separate compensators, (1)" a""1i`a`t compensator whichelfects an amplication change which is flat `or non-discriminat'ive with respect to frequency `and (2,),v a twist or slope compensator which m'aylmbe designed to effect greater amplification changes` at the'upper than at'v the lower frequencies or it may be designedtoeiect greater changesatthenv lower than at the upper frequencies. Suchrcoinpensatorsmay be associatedwith one or more of the' amplifiers or repeaters which forma part of the" transmission system'v and `may' be automatically operatedby'means of pilot transmissions.` Thus a pilot transmission at a frequency at the lower edge ofthe transmitted band Hmay be sent over t'l'iecircuit, picked off 4at a Y.convt'enient point or pointsfrectified and: the rectified current; employed `to4 operate a motor geared or otherwise coupld'to thev appropriate confipensatorwhichy may bea -potentiometer,- an attenuatorA-Aornother means for changing'gainin equal amounts at allA frequencies. Aconvenient arrangement involves picking off the "pilotftransmission at Ythe output of' an amplifier and using 4the rectified-currentsupplied motor-driven compensator to effect a change in the gainand thus the output' level of that amplifier at the frequency ofthe pilot transmission until aspec'ified level is reached. The flat compensator is then prODelf 1y Set.

In like manner, a second pilot transmissionat a frequency at the upper edge of the transmitted band may be employed/to set theslopeor twistV compensator. This general Vmethod employingl two"'pi1ots is vdisclosedin patent to K. kC.,Bla,ck

2,019,594', November', 193,54and another use o f nat and .twist or slope rcompensatorsis ,disclosed in l F. @Brooks patent 2,075,975, April c, `193,7.

My'invention incorporates various improvements in the regulation o f board bandcarrier `transmission systems, including means whereby the setting for each` pilottransmission is substanf tially independent of the adjustment'of the other, also Ameans for lthe prevention of amplier overloadkin the case of abnormal line attenuationar and various features inter-related thereto. For a morezcomplete understandingnof these matters, referenceV is made to thev detailed`description taken together with the drawing in which:

Fig'. 1 shows a schematicxdiagram of a -regulating repeater circuit according 'to the' invention;

*.rig.4 2 illustratesthe` attenuation` l frequency characteristics of"th`e"-networks 'which may be employedin a slope regulator, and` Fig. 3 is a table in which are tabulated figures illustrative of the method of functioning of the circuit of Fig. 1.

Before proceeding with a detailed description, it should be noted that Fig. 1 shows a one-way repeater circuit for insertion in a multiplex line at an intermediate repeater point, and that this repeater comprises an amplifier of three sections in tandem, viz., I6, 30 and 3| connected between the directional filters I and 32 with gain control means partly ahead of the amplifier and partly between sections I6 and 30.

Fig. l may be considered to represent diagrammatically a means of practicing the invention by the employment of a special repeater designed particularly to provide regulation to compensate for severe weather conditions. Its input terminals 31, 38 are connected to a receiving directive filter I. This filter for purposes of illustration may be assumed to pass a band of frequencies including a lower limiting frequency of 92 kilocycles and an upper limiting frequency of 140 kllocycles.

The receiving directive filter I may be assumed to pass the band 92 kilocycles to 140 kilocycles with a uniform attenuation of '7 decibels and substantially prevent the passage of all other frequencies. The output of filter I is connected to 10-decibel pad 2 which provides a constant impedance for the termination of filter I and transformer 3. Thus transformer 3 which may have an impedance of 125:20,000, presents a comparatively high impedance substantially independent of frequency against which the double .potentiometer 4 may function. An impedance ratio of 125:2(),000 represents a Voltage step-up of approximately 22 decibels.

The potentiometer 4 consists of two three-plate condensers in tandem, the combination arranged lto function in the manner of a single three-plate condenser as used and discussed in the'patents to Anderson et al. 2,106,336, January 25, 1938, and C. R. Eckberg, 2,115,141,April 26, 1938. The first condenser comprises fixed plates 5, 'I and movable plate 6 and is so designed that the sum of the capacity between plates 5 and 6 and the capacity between plates 'I and 6 is a constant for any position of the movable plate 6. The second condenser comprises fixed plates 8, I and movable plate 9 and is so designed that the `sum of the capacity between plates 8 and 9 and the capacity between plates IIJ and 9 has a constant value for any position of movable plate 9. Each condenser performs as a potentiometer having an infinite number of steps and by the tandem connection shown, structural advantages accrue such as small capacity values combined with rugged construction.

It may be observed that since the line voltage, incoming through filter I and pad 2 and stepped up through transformer 3, appears, looking to the. left at D-D, as across a substantially pure resistance, the condenser 4, which preferably should have an impedance which is large in comparison with said resistance, functions as a potentiometer which is substantially flat with frequency when operating into the grid circuit of amplifier I6. The condenser type potentiometer is operated through the drive shaft I'I by the pilot motor control II, and in a preferred embodiment may cause attenuation of minimum 6 decibels, maximum 47 decibels.

In series with the condenser potentiometer 4 and the grid circuit of amplier I6 is the condenser I2 comprising the fixed plates I4, I5 and the movable plate I3. Condenser I2 is operated by pilot motor control I9 through drive shaft I8 which is extended to operate multiplate condenser 2U. In the shown position, movable plate I3 of condenser I2 is out of mesh with stationary plates I4 and I5 and the direct capacity between plates I4 and I5 is of such a value as to constitute negligible loss in the connection between condenser potentiometer 4 and the grid circuit of amplifier I6 in view of the high impedance of the said grid circuit. When, however, movable plate I3 is operated so as to mesh between stationary plates I4 and I5, it functions to shield plate I4 from plate I5 thereby reducing the direct capacity between them and at the same time introducing a direct capacity to ground through lead 50 from each stationary plate respectively. Thus, for that portion of the. motion of movable plate I3 in which it is meshed to a greater or less degree between stationary plates I4 and I5, condenser I2 constitutes a variable network of ll section as disclosed in E. B. Payne Patent 2,075,957, April 6, 1937, and is there demonstrated under the conditions shown to constitute a fiat attenuator, that is, free from frequency discrimination.

The condenser 20 is a multielectrode or multiplate condenser, which may have a large number of stationary plates or electrodes. In the shown embodiment there are eight stationary plates, 2|, 22, 23, 24, 25, 26, 21, 28 and a single movable electrode 29. The fixed plates are shown as arcs of a circle concentric with the circle of which the movable plate 29 is an arc. In practice, these plates would take the form of one or more circular sectors as is common in variable air condensers. If desired, either the fixed plates or the movable plates, or both, may be specially shaped in order to produce a desired relation between angular movement of the movable plate and change in electrostatic capacity. In case condenser` 20 is Aconstructed with eight fixed plates, each plate would occupy approximately 45 degrees of arc and the movable plate 29 would also preferably in this case span approximately 45 degrees of arc.

It should be noted that the coupling between condensers I2 and 20 by means of drive shaft I 8 is so phased that movable plate I3 of condenser I2 is in a position of maximum coupling or full mesh with stationary plates I4, I for that position of shaft I 8 which brings movable plate 29 of condenser 20 into maximum coupling or full mesh with stationary plate 28, and thus, if the condenser 20 has eight fixed electrodes as shown, the plates I3, I4, I5 of condenser I2 will preferably each span approximately 45 degrees of arc.

` The output of amplifier I6 which may be designed to have affixed gain of 21 decibels is shown connected by leads 49, 50 to networks I to VII in parallel. These networks may be T sections as shown but will ordinarily contain a plurality of both reactive and non-reactive components, and thus the attenuation through them will vary with frequency.

The series of networks shown are assumed to be so designed as to have equal attenuation for the lowest frequency of the band to be transmitted, for example, 92 kilocycles and to have an attenuation which departs increasingly from the low frequency or 92 kilocycles attenuation as the frequency is 'increased up to the highest band frequency, for example 140 kilocycles. Moreover, the series of networks is so designed with respect to each other that theamount of'said departure atagiven frequency changes progressively from network tol network throughout thek series. This condition is illustrated by the curves ofj Fig. 2, inwh'ich, whereas all networks are shown to havean attenuation of 23 decibels at 92 kilocycles, network I has at 140 kilocycles an attenuation of- -l decibel, network II +10 decibels, network IIIv +18 decibels, network IV +24 decibels, network V +27 decibels, network VI +30 decibels and network VII +433 decibels. The curves of Fig. 2 are plotted in terms of the algebraic change in-V transmission level at the various frequenciesl in passing through the respective networks.-

The outputsofthe several networks I to VII are connected tothe stationary electrodes 2i to 2l', respectively, of thelmultielectrode condenser 20, and thus the condenser 20- functions as a combined coupler and potentiometer such that any characteristic of frequency discrimination between that represented by networkv I andthat represented by network VII may be inserted by an appropriate setting of the movable electrode 29. This general method for obtaining intermediate characteristics was disclosed in E'ckberg, supra, as applied to a-v single-network` and as eX- tended to two'or more-networks in R.. W. Chesnut Patent 2,049,195, July 23, 1938.

It is apparent that the combinationy of the networks I to VII with the condenser 29, the movable electrode of which isconnected by leady 5i to the gridcircuit of amplifier 39, constitutes a twist or slope compensator which, properly motivated, iscapable of inserting a regulatory effect of any required characteristic betweenthat of curve I and that of curve VII of Fig. 2.

The amplier Sil is fundamentally a part of the slope compensator circuit, is inserted to provide a high impedance termination to the multielectrode condenser coupler 29, and may have a net gain of 4 decibels.

Amplifier 3i contributes the main amplification of the repeater. It may be of the feedback type disclosed inl Ii.V S. Black. Patent 2,102,671, December 2l, 1937, having a frequency discriminative element in its feedback or circuit (not shown). Thus it may produce a gain of 44 decibels at 92 kilocycles and 53 decibels at 140 kilocycles.

'Ihe transmitting directivefilter 32 may be designed to'pass the band 92 kilocycles to 140 kilocycles and may be assumed to have negligible attenuation throughout theband.

It is assumed that 92-kilocycle and l40-kilocycle pilots are picked off the circuit at the output of the amplifier 3i. The 92-kilocycle wave is carried by leads 59, 69 to 92-kilocycle selective lter 58, thence to amplifier-rectifier 5l'. and smoothing nlter 59 and so by leads 35, 36 to pilot motor control l. Similarly the l40-kilocycle wave is carried by leads 6 I, 32 to l40-kilocycle selective filter 54, thence to ampliier-rectiiier 53 and smoothing filter v52 and so by leads 3&3 to pilot motor control i9. For the sake of simplicity the details of the various selective, amplifying and rectifying circuits and the details of the motor drive have been omitted, since the general practice of pilot control is old and well known to those skilled in the art, capable of accomplishment in various ways, and no novelty in the means of securing pilot operation of control devices is claimed in this invention.

rThe operation of' a regulating repeater according to the invention as eniployedfor thecompensation of variations in weather conditions on an open-wire line carrying a broad-band' of carrier transmission in accordance with a preferred embodiment as shown in simplified diagram in Fig. l may be clarified by a consideration of its operation under a number of assumed typical line andl weather conditions.

For the purposes-of illustration it is assumed that the line in question is equipped with a plurality of repeaters spaced at appropriate intervals throughout its length, that one or more of said repeaters may be specifically designed to compensate for severe variations in weather conditions,` of which special repeaters, the repeater of Fig. i, may be assumed to be illustrative, and

that the general theory of operation of the system is based-on the proposition that the output transmissiony level at a repeater point shall have been adjusted through, or by reason of, the repeater equipment, to` a predetermined value, said value being identical for all frequencies. This state of transmission levels may, under practical conditions, not be precisely achieved in every case but, it may be assumed to be an intrinsic basis of repeater design. Thus, by way of illustration,.it may be assumed that under ideal conditions, repeater output will be at zero level at all frequencies and hence repeater input level at` each frequency will represent the attenuation produced at the particular frequency by the section ofiine following the last preceding repeater.

The general` procedure of compensation to be described comprises adjusting potentiometer i under the control of the 92-kilocycle pilot to give the Vproper output level at the-9-2-kilocycle frequency and regulating the interstage compensator between stages iii and l-to give the proper slope over the 92 to l40-kilocycle band under the control of the 140-kilocycle pilot.

Referring to the table of Fig. 3, row l may be assumed to present a typical state of the repeater when used on a shortv dry line, that is, on a line onv which the distance to the next preceding repeater is comparatively short, the line being for that distance assumed to be surrounded by an atmosphere of low relative humidity. rIhus, letI input level at A-A be at 92 kilocycles, -4 decibels and at 140 kilocycles, -5 decibels. Having passed through receiving selective filter l, these levels are reduced at B-B to il decibels and 12 decibels, respectively, arefby the pad E reduced at C-C to -21 decibels and 22 decibels, respectively, and by reason of the voltage stepup of the transformer 3 appear at DWD at levels of +1 decibel` and zero respectively. This is a high level for the input to condenser potentiometer l and the low frequency 92-ki1ocycle pilot actuates the pilot motor control l l to cuiI in a at loss of 47 decibels, the greatest loss of which potentiometer 4 is capable. No loss is assumed to have been` introduced by condenser i 2 and thus the level at E-E becomes +46 decibeis at 92 kilocyclesand 47 decibels at 149 kilocycles, which is reduced by amplifier i 6 to values at F--F of -25 decibels and 26 decibels. In passing through'theslope compensator comprising the networks I to VII and the multielectrode coupler 2U, the level at 92 kilocycles is reduced by the minimum attenuation of the slope compensator, i. e., 23 decibels to a level of 48 decibels at G-G whereas the l40-kilocycle pilot causes the operation of pilot motor control i9 to position movable `electrode 29 at a point intermediate between full coupling with iixed electrode 29 and full couplingwith xed electrode 2l so that an attenuation of 31 decibels at 140 kilocycles is introduced thus bringing the kilocycles level at G-G to 57 decibels. The amplier 30 introduces a flat gain of 4 decibels thus raising the level at H-I-I to 44 decibels at 92 kilocycles and 53 decibels at 140 kilocycles, and in sequence the feedback amplifier 3|, adding its gain of 44 decibels at 92 kilocycles and 53 decibels at 140 kilocycles brings the repeater output at I-I to Zero level for all frequencies since, as noted above, the transmitting selective iilter 32 is assumed to introduce no loss.

Row 2 of the table of Fig. 3 illustrates the adjustments which mig-ht be found in the repeater of Fig. l when used in sequence to a long wet section of line. In this case the incoming levels might be 28 decibels at 92 kilocycles and 35 decibels at 140 kilocycles which are reduced to 23 decibels and 30 decibels at D-D the input to the fiat compensator. This compensator introduces a flat loss of 23 decibels which, combined with the 21-decibels gain due to amplifier IS, results in levels at F- F the input to the slope compensator of 25 decibels at 92 kilocycles and 32 decibels at 140 kilocycles. The slope compensator introduces only 2 decibels slope loss over its xed flat attenuation of 23 decibels. Thus the loss at 140 kilocycles is only 2 decibels more than at 92 kilocycles and the levels at G- G are 48 decibels at 92 kilocycles and 57 decibels at 140 kilocycles which the amplifiers 30 and 3| bring down to zero level as before. In this case, the pilot motor control actuated by the 140-kilocycle pilot positions the movable plate 29 of multiplate condenser 20 at a position overlapping both fixed plates 24 and 25.

Consideration of the operation of the repeater self-adjusting 'compensato-rs as outlined above, emphasizes a major advantage of my invention. In devices of the sort as used heretofore, the slope or twist compensator introduced not only a variable high frequency adjustment but a variable low frequency adjustment as well. Any change in the compensation for high frequency loss was accompanied inherently by a change in the low frequency compensation which thus would ordinarily require a readjustment of the slope compensator, and so on. This condition arose from the fact that the networks producing slope attenuation were all in series, and the final result was the sum of the component results of the individual networks.

Now I have discovered that it is possible to design a series of networks each one of which is capable by itself of displaying the desired characteristics of' a portion of the required operating range, and of having such electrical proportions that when all the networks are connected in parallel a iiat and invariable loss over the whole frequency range results, to which may be added the frequency discrimination attenuation of the particular network. Thus I am able to obviate the cut-and-try method of adjustment and to adjust the fiat gain once and for all by means of the flat compensator so that the l40-kilocycles control of the slope compensator adjusts only slope and has no effect upon the 92-kilocycles gain. Thus my invention affords improved stability and reduced maintenance.

A further important advantage of my invention lies in its provision for the prevention of overload as will be apparent from a consideration of rows 3 and 4 of the table of Fig. 3. It is assumed in this case that by reason of abnormal conditions in the line and equipment preceding the repeater under consideration, the input level at the low frequencies, as for example at 92 kilocycles, has been greatly reduced. Thus the level at A- A may be 40 decibels at 92 kilocycles and 5 decibels at 140 kilocycles. Considering row 3 which illustrates what would happen if condenser I2 were not included, the fiat compensator introduces an ll-decibel loss and the level at F-F becomes 25 decibels at 92 kilocycles and -|-10 decibels at 140 kilocycles. It will be noted that the level at F- F is the same as for rows I and 2 at 92 kilocycles but is much higher at 140 kilocycles. The slope compensator is therefore operated to insert maximum loss, namely, 33 decibels which corresponds to complete coupling between movable electrode 29 and fixed electrode 2l which, however, only reduces the 140-kilo.- cycle level to 22 decibels at G-G and this level plus the amplification of amplifiers 3U and 3| would become an output of +35 decibels. Amplifier 3| would ordinarily be unable to handle the power represented by this level and overload would result. It is to obviate a condition of this sort that I have devised the variable condenser network I2 operated by the high frequency pilot. The operation of this network is shown by the tabulation of row 4 of the table of Fig. 3. Assume, as before, that the 92-kilocycle pilot acting through pilot control motor I I will cut down the flat level to the point where Zero level at 92 kilocycles is produced at the output. The' output level of the fiat attenuator is thus 46 at 92 kilocycles and 11 at 140 kilocycles. Because of the high level of the 140 kilocycles the pilot control motor I9 will continue to operate beyond complete coupling between movable plate 29 and iiXed plate 21 and will couple movable plate 29 with fixed plate 28 and at the same time mesh movable plate I3 between fixed plates I4 and I5 of condenser network I2. This motion will continue as long as the output level of the lfl-kilocycle pilot is above zero level. Since the network I2 is a flat network, its operation is to reduce the 92-kilocycle pilot as well as the 140-kilocycle pilot and hence as it increases attenuation, iiat regulator 4 will decrease attenuation in the effort to maintain the output level of the 92-kilocycle pilot at zero level. At the completion of this process the 140-kilocycle pilot is reduced to a zero level output, the regulator 4 has been adjusted to minimum loss, namely, 6 decibels, and the network I2 has inserted a loss of 40 decibels, making a total fiat loss between D--D and E-E of 46 decibels. Thus the level at E-E is 81 decibels at 92 kilocycles and 46 decibels at 140 kilocycles. The amplifier I6 brings these levels up to 60 decibels and 25 decibels at F-F. Since in order to bring the 140-kilocycle pilot to Zero level at the repeater output, the movable plate 29 of coupler 29 has been positioned substantially opposite fixed plate 28, which, however, is coupled to plate 2l, there is introduced between F-F and G-G the loss of network VII which is 23 decibels at 92 kilocycles and 32 decibels at 140 kilocycles, so that the level at G -G becomes 83 decibels at 92 kilocycles and 57 decibels at 140 kilocycles and hence adding the gain of amplifiers 30 and 3| the output level at 92 kilocycles is 35 decibels and at 140 kilocycles, Zero or normal level. Thus overload of the amplifier is prevented and subsequent repeaters and line sections with their normally greater attenuation at the upper than at the lower frequencies will function to progressively correct the discrepancy between the high and low frequency levels.

I have thus tracedstep by step the operation of a specic embodiment of my invention employing for that purpose a particular association of apparatus and particular numerical values. This arrangement, however, isY not to be considered to circumscribe or limit the said invention, it being understood that various modifications and divergences may be incorporated. In particular, it is to be noted that whereas in the illustrative embodiment I have shown the high frequency pilot in control of equalization or slope and the low frequency pilot functioning to control flat gain, this arrangement may be reversed and the upper pilot used to control fiat gain, the lower pilot to control slope. In this case I would ordinarily design the network characteristics to have a negative rather than a positive divergence so that the several characteristics would be shown fanning out to the left instead of to the right as in Fig. 2.

Not only may the functionsof the upper and lower pilots be interchanged as between regulation of flatgain and slope equalization but these functions under some circumstances may with advantage be divided between them and thus each pilot may control the insertion of a network (or a fraction thereof) by which both flat gain and slope are simultaneously affected. Since both pilots will, in general, function simultaneously and any operation by one pilot may induce resultant operation by the other, care must be exercised that an unstable arrangement is not produced.

I have found that for stability either pilot producing a given change at one pilot frequency must, if it is itself of that frequency, produce a lesser corresponding change at the other pilot frequency than would be produced by the other pilot under the same conditions. As an example, a decibel change at the lower pilot frequency caused by the lower pilot must be accompanied by a lesser change at the upper pilot frequency than would accompany a decibel change at the lower pilot frequency caused by the upper pilot, and similarly a decibel change at the upper pilot frequency caused by the lower pilot must be accompanied by a greater change at the lower pilot frequency than would accompany a decibel change at the upper pilot frequency caused by the upper pilot.

These and other modifications may be made without departing from the spirit of the invention as defined in the appended claims.

What is claimed is:

l. A transmission regulating system, comprising a first regulator, adapted to produce increments of transmission level change in identical amounts at all frequencies, and adapted to be operated in response to a first pilot frequency, a second regulator adapted to produce increments of transmission level change in amounts which are a function of the frequency of transmission and adapted to be operated in response to a second pilot frequency, and an emergency regulater adapted to produce increments of transmission level change in identical amounts at all frequencies under the independent control of said second piiot frequency.

2. A transmission regulating system for a carrier wave transmission line transmitting carrier transmission over a Wide band of frequencies, comprising a first pilot frequency at the lower edge of said band, a second pilot frequency at the upper edge of said band, means for the control of transmission level over said line in small increments of equal amount at all frequencies throughout the said band actuated by said first pilot frequency, independent means for the control of transmission level over said line in small increments but said increments a predetermined function of frequency actuated by said second pilot frequency, and emergency means actuated by said independent means adapted to provide A frequency discriminative variations in line attenuations, and auxiliary means actuated only by said second means adapted to prevent overloading of said amplifier in the presence of abnormal frequency discriminative variations in line attenuations.

4. A transmission regulating system for a carrier wave transmission line transmitting a wide band of frequencies, comprising an amplifier, a rst means adapted to regulate the gain of said amplifier in a manner to compensate for average variations in line attenuations comprising a first attenuator controlled by a pilot wave at a frequency having comparatively low line attenuation, a second means adapted to regulate the gain of said amplifier in a manner to compensate for frequency discriminative variations in line attenuation comprising a second attenuator controlled by a pilot wave at a frequency having comparatively high line attenuation and auxiliary means controlled exclusively by said second means adapted to supersede the control of said second attenuator to prevent amplifier overload.

5. The method of regulating amplication over a line transmitting a broad band of frequencies Which consists in transmitting a first pilot wave at a frequency at one edge of the band, transmitting asecond pilot Wave at a frequency at the other edge of the band, adjusting the average line gain on the basis of the received level cf said rst pilot adjusting the line gain differentially as to frequency on the basis of the received level of said second pilot, and preventing overload by additional flat regulation on the basis of the resulting level of said second pilot after said adjusting of average line gain on the basis of the received level of said first pilot.

6. The method of compensating for changes in attenuation varying with frequency over a line transmitting a .broad band of frequencies which consists in transmitting over said line a first pilot wave at a frequency of low change in attenuation, transmitting over said line a second pilot wave at a frequency of high change in attenuation, providing an excess of line amplification and reducing the average amplification to hold the received level of said first pilot to a predetermined value, changing the ratio of low frequency amplificati-en to high frequency amplification to hold said second pilot to a predetermined level and providing an .additional flat gain reduction by said second pilot to prevent high frequency overload.

7. In combination, a series of networks having attenuations which are substantially a linear function of frequency over a given frequency range, khaving denticalattenuations at the lowest frequency of said range and having progressively greater change in attenuation with increase of frequency from network to network throughout the series, the inputs of said networks connected in parallel, and means for supplying thereto Waves of the said range, the outputs of said netn works respectively connected in the order of the series to stationary plates of a variable air condenser which comprises a series of equally spaced stationary plates without substantial mutual capacityY from one plate to another, and a movable plate adapted to be set from full electrostatic coupling with one stationary plate to any division of coupling between two adjacent plates, an amplier and Vmeans for making electrical connection from said movable plate to the input of said amplier, in series in the input circuit of said amplier an adjustable flat attenuator having stationary and movable elements so related that for the greater part of the range of movement of said movable element the attenuation is substantially small and constant while for a small part of said range the attenuation increases rapidly, a fixed mechanical connection between the movable element of said flat attenuator and the movable plate of said air condenser tying the motion of .one to the motion of the other and said xed connection so assembled as to maintain a desired sequence relation between the respective positions of said movable plate and said movable element whereby the gain-frequency characteristic of said amplier is regulated without substantial change in at gain for normal amplifier inputs but whereby flat attenuation is cut in circuit to prevent overload of said amplifier.

v8. The combination of claim 7 in which said adjustable at attenuator comprises a T-network having capacity elements.

9. The combination of claim 7 in which said adjustable flat attenuator comprises an air condenser having two fixed plates and one movable plate.

10; The combination of claim 7 in which said movable plate is driven by a motor and operating power therefor is supplied by a pilot wave within the said frequency range and selectively drawn from the output of said amplifier.

ROBERT S. CARUTHERS. 

